The present invention relates to a hydraulic cylinder having a piston rod bearing a tool-receiving element.
Usually, because of the pressing force required, hydraulic drives having hydraulic cylinders are used for mechanical deforming or joining processes, such as for example punching, stamping, flanging, crimping, riveting or clinching. For safety reasons, when the power stroke of hydraulic cylinders is more than 6 mm, great safety requirements are placed on the systems used. Thus, for avoiding risks, complicated protective curtains and/or mechanical, pneumatic or electrical interlocking devices must be provided. As a result, the operator is very restricted in “handling” work in the dangerous region of the ram of a corresponding deforming or joining device.
Hydraulic cylinder systems which shut off the flow of the hydraulic medium in the hydraulic cylinder by an additional control circuit are well known. Thus, for example, a press is known in which the power stroke is turned on only when an electrically conducting connection is procured through the structural part to be worked via a ram of a tool-receiving element. Here, “finger safety” is secured because of the non-conductivity of the finger or fingers. Additional devices are known in which two electric sensors, working independently of each other, ensure that only a 6-mm power stroke takes place.
Known in addition is a press which uses an internal path-measuring system in order to allow, after calibration, the power stroke to be reduced or first set to 6 mm. Other accomplishments consist for example of a hinged mechanism, covering the region endangering the hands of the operator and only allowing the power stroke when in the folded state. Likewise known is a device with powerless partial thrust, which is characterized in that the drive of the thrust used therein is effected pneumatically and is limited to a total stroke of 60 mm. Another accomplishment consists in avoiding violation of the safety distance of 6 mm by limiting the speed of thrust to a prescribed amount, in order to ensure timely removal/withdrawal of the operator's limbs.
It is the object of the invention to propose a hydraulic cylinder which avoids the complicated and costly precautions known in the prior art as much as possible.
This object is accomplished by a hydraulic cylinder as described herein. By this means, an additional control circuit, be it pneumatic, electrical, mechanical or a combination thereof, advantageously becomes superfluous. The device, with like operating safety, thereby becomes substantially less costly than known systems. Here, the medium (e.g. hydraulic oil) which is used for the development of power is at the same time the medium for the safety shut-off. Other media, such as water or the like, for example, are alternatively usable here. An additional advantage consists in that the device according to the invention is not limited to a total stroke. Theoretically, all desired strokes technically producible can be executed. In addition, the device according to the invention also has the advantage that it is not limited to a maximum speed of movement. The thrust as well as the power stroke speeds are freely selectable with use of the device according to the invention, provided that the technical provisions (existing volumetric flow, feed-line cross sections, etc.) permit this. In addition, with the device according to the invention an internal anti-twisting means advantageously can be integrated with the device according to the invention and via a continuous piston rod an external adjustable full-stroke limitation can be built on, which makes it possible to limit the return stroke of the piston/ram, and hence the full stroke, in any desired position, in simple fashion.
The safety or automatic shut-off means is provided with two valve slides working independently of each other, i.e., upon failure of a component, a forward movement of the ram which might be a danger is no longer possible.
For better understanding of the invention and in order to show how it can be executed, it is described briefly below with reference to an exemplary embodiment.